Method and system for measuring depth to saturated soils

ABSTRACT

A method is provided for determining the depth to the saturated soil at a selected site. A first and second instance wherein the depth of the water table equals the depth to the saturated soil is identified ( 142 ). The rate of change of the depth to the saturated soil is calculated ( 146 ) as a function of the specific yield, amount of time between the instances, total precipitation occurring between the instances, and depth of the water table at each instance. Then, the depth to the saturated soil for a third instance is calculated ( 148 ) as a function of the rate of change of the depth to the saturated soil. The calculated depth to the saturated soil for a third instance may be adjusted as a function of precipitation occurring at the site.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of prior U.S. patent applicationSer. No. 10/198,056, filed on Jul. 17, 2002, priority from the filingdate of which is hereby claimed under 35 U.S.C. § 119.

FIELD OF THE INVENTION

The invention relates to methods of determining the depth to saturatedsoils and more particularly to using that information to determinewhether a particular site is a wetland.

BACKGROUND OF THE INVENTION

Wetlands may include marshes, bogs, and swamps. Wetland delineationtends to be controversial because such a determination often pits theinterest of environmental protectionists against the interests oflandowners. Therefore, standards or guidelines have been created tostandardize wetland delineation. These guidelines also attempt tobalance the interests of the public and private landowner. According totypical guidelines, whether a particular parcel of land qualifies as awetland generally depends upon the percentage of the growing season thatthe surface of the soil is continuously saturated with water.

One example of wetland delineation guidelines includes the Corps ofEngineers' Wetlands Delineation Manual of January 1987 (“'87 Manual”).The '87 Manual provides guidelines that may be used to determine whethera particular parcel of land is a wetland. Generally, land qualifies as awetland if it is continuously saturated to the surface between 5% and12.5% of the growing season. However, the '87 Manual indicates that manysites are not wetlands despite being continuously saturated to thesurface between 5% to 12.5% of the growing season. A delineation inthese cases is left to the judgment of the delineator. According to the'87 Manual, sites not continuously saturated at least 5% of the growingseason are not wet enough to be considered wetlands.

The '87 Manual provides that, delineators (persons who determine whethera site is a wetland) may consider three parameters, soilcharacteristics, vegetation, and hydrology, when evaluating whether asite is a wetland. Soil characteristics may be used to determine whetherthe soils at the site are hydric soils. Hydric soils form underconditions of saturation including flooding that persists long enough todevelop anaerobic conditions in the soil. These anaerobic conditions,characteristic of hydric soils, may be observed as color changes in thesoils.

The hydrology determination, i.e., the depth to saturation in the soil,is the most controversial determination because a delineator cannotdirectly observe the hydrologic condition of the ground. Therefore, thedelineator must rely on other indicators such as vegetation and soilcharacteristics to make the hydrology determination. Accuratelyevaluating the site in this manner requires numerous visits to the site.However, it is not uncommon for a delineator to make a delineation basedon only a single visit. After the hydrology determination is made, itmay be compared with standard hydrological criteria, such as those foundin the '87 Manual.

Digging a pit in the ground and measuring the depth at which waterappears in the pit will yield the depth of the water table (the upperboundary of a free groundwater body at atmospheric pressure). However,the depth of the upper boundary of saturated soil. (referred tohereafter as the depth to the saturated soil) is not necessarily equalto the depth of the water table. Capillary action, described in moredetail below, may draw water up through the grains of soil to a levelabove the water table causing saturated soil to occur above the watertable. The volume of water located between the depth to the saturatedsoil and the water table is known as the capillary fringe. Both thedepth to the saturated soil and the water table may rise during rainfalland shrink when depletion mechanisms such as drainage, evaporation, andtranspiration deplete water from the soil.

Referring FIGS. 1A–1E, the capillary fringe will be explained in greaterdetail. FIG. 1A depicts a barrel 10 of soil grains 14 and water 12. InFIG. 1A, the water 12 extends above the top surface of the soil grains14 and water 12. In FIG. 1A, table 16 extends above the surface of thesoil, there is no capillary tension. FIG. 1B depicts barrel 10 afterwater 12 has been depleted from the barrel 10 to the point that thedepth to the saturated soil and the water table 16 are at the top of thesurface of the soil grains 14. Therefore, there is no capillary fringe.

FIG. 1C depicts barrel 10 after one additional drop of water has beendepleted from the barrel depicted in FIG. 1B. Menisci 20 form betweensoil grains 14 at the surface of the soil. As can be seen in FIG. 1C,the water table 16 has dropped to well below the surface of the soilwhile the depth to the saturated soil remains at the surface of thesoil. Each menisci 20 has water 12 on one side and air on the other.Because the water 12 is attracted to the soil grains 14, the water 12relentlessly seeks to encompass more soil grains 14. Capillary forcesdraw the water 12 upward to the surface of the soil forming a zone ofnegative pressure, known as a capillary fringe 22, between the depth tothe saturated soil and the water table 16. The surface of the capillaryfringe 22 is formed by menisci 20. The surface of the capillary fringe22 may also be referred to as the capillary front. The capillary fringe22 is depicted as a gray area between the water table and the surface ofthe soil. The capillary fringe 22 will move upward until negativepressure behind it reaches the maximum the menisci 20 can support. Inthis manner, the capillary forces create a pressure differential acrossthe menisci 20 between the saturated soil and the air above the menisci20. Above the menisci 20, the pressure is atmospheric. Below the menisci20, the pressure may be as low as minus 12 inches of water. The negativepressure along the surface of the soil makes a visual observation of thedepth to the saturated soil difficult because the surface of the soilmay appear dry despite the fact that the soil grains directly underneaththe surface grains are fully saturated with water.

FIG. 1D depicts barrel 10 after an additional drop of water has beendepleted. In this figure, the water table 16 has fallen to the maximumdistance the menisci 20 will support. This is evidenced by the fact thatair entry 26 has occurred at the surface of the soil. Because thenegative pressure between the menisci 20 and the water table is at itsmaximum, as the water table drops, it will pull the depth to thesaturated soil downward with it. Under the conditions depicted in FIG.10, the capillary fringe 22 is at its maximum length, which may be aslarge as approximately 12 inches of water. Again, the negative pressurealong the surface of the soil will make the soil appear dry despite thefact that ground remains saturated to the surface of the soil.

Further water depletion from the barrel, as depicted in FIG. 1E willproduce a non-saturated condition at the surface of the soil. Underthese conditions, the depth to the saturated soil (i.e., depth to thesurface of the capillary fringe 22), and the water table 16 will changedepths at the same time separated by the full extent of the capillaryfringe 22 (up to 12 inches).

Specific yield is the fraction of the saturated soil consisting of waterthat will drain by gravity when the water table drops. The magnitude ofthe drop in the depth to the saturated soil from FIG. 1D to FIG. 1E is afunction of the specific yield of the soil contained in barrel 10.

In FIGS. 1A through 1E, as water was depleted from the barrel 10, noadditional water was added. Of course, this is generally not the case inthe field. When water is added, such as by precipitation, the surface ofthe capillary fringe becomes disturbed when new water fills in themenisci and relaxes the tension between the surface of the capillaryfringe and the water table. When the tension is relaxed, the pressure inthe area of negative pressure increases to atmospheric and the watertable moves upward to the depth to the saturated soil. Under thesecircumstances, the water table and saturated soils may occur at the samedepth which may be above, at, or below the surface of the soil. When therain ends and the excess surface water has been depleted, the volume ofwater in the saturated soil decreases restoring the tension between thedepth to the saturated soil and the water table.

Referring to FIG. 2, an idealized hydrograph 40 of a wetland can beviewed. Arrow 42 depicts time increasing from the left-hand side to theright-hand side of FIG. 2. In this figure, the capillary fringe 50 spansthe distance between the surface of the soil 44 and the water table 48.A period of rain starting at time 54 and ending at time 56 causes thewater table 48 to move upward to a location above the surface of thesoil 44 creating a volume of water 58 above the surface of the soil 44.At this period of time, the capillary fringe collapses. After the periodof rain, water is depleted. Water may be depleted by depletionmechanisms such as drainage, evaporation, and/or transpiration. Whensufficient water has depleted, the water table 48 drops in depth and thecapillary fringe 50 is reformed. However, the depth to the saturatedsoil 46 remains at the surface of the soil 44 with the capillary fringe50 spannning the distance between the surface of the soil 44 and thewater table 48. Therefore, FIG. 2 depicts a site where the surface ofthe soil is continuously saturated with water such as certain types ofwetlands.

Referring to FIG. 3, an idealized hydrograph 80 of an upland site may beviewed. Arrow 82 depicts time increasing from the left-hand side to theright-hand side of FIG. 3. Initially, both the water table 88 and thedepth to the saturated soil 86 are separated by the capillary fringe 90.Both surfaces continuously drop until the start 94 of a period of rainwhen the capillary fringe 90 collapses and both the depth to thesaturated soil 86 and the water table 88 rise to the surface of the soil84. As water is added, surface water 98 is appears above the surface ofthe soil 84. Shortly after the end 96 of the period of rain, water isdepleted by depletion mechanisms such as drainage, evaporation, and/ortranspiration. As a result of the depletion of water, the water table 88drops and the capillary fringe 90 is reformed. The capillary fringe 90maintains the depth to the saturated soil 86 at the surface of the soil84 only briefly. As the water table 88 drops, the depth to the saturatedsoil 86 also drops. Therefore, in this idealized depiction of uplands,the depth to the saturated soil reaches the surface of the soil onlyduring periods of precipitation and for a short time thereafter.

In summary, the capillary fringe causes the depth to the saturated soilto be as much a 12 inches above the water table. As mentioned above, thedepth of the water table can be determined by the depth at which wateroccurs in a pit dug into the ground. Such a pit is typically 18 inchesdeep. However, the depth to the saturated soil cannot be accuratelydetermined in this manner because the membrane of menisci and soilgrains forming the surface of the capillary fringe may follow the wallof the pit holding water in the soil and preventing water from enteringthe pit. Under these circumstances, water will not seep into the pitfrom the saturated soil so long as the negative pressure of thecapillary fringe opposes the seepage of water into the pit. Furthermore,the negative pressure along the pit wall obscures visual observation ofthe saturated soils (i.e., the soil along the pit wall adjacent to thesaturated soils will appear dry despite the fact that the soil behind itis saturated with water). Determining which sites are wetlands istherefore complicated because determining the depth to the saturatedsoil is difficult.

One method of directly measuring the depth to the saturated soilinvolves using a tensiometer to measure suction or tension in the soil.This method works by using the tensiometer to determine the depth atwhich the pressure in the soil changes from atmospheric to belowatmospheric (i.e., negative pressure). However, this method has thedrawback of requiring blind searching to locate the depth to thesaturated soil.

Determining the depth to the saturated soil is useful in determiningwhether a site qualifies as a wetland. The depth to the saturated soilcan be used to determine the duration of continuous saturation at thesurface of the soil. The duration of continuous saturation at thesurface of the soil can be compared with wetland standards, such asthose found in the '87 Manual, to determine whether a particular siteshould be considered a wetland.

It is clear from the above discussion of the capillary fringe that adetermination of the depth of the water table alone is insufficient todetermine the depth to the saturated soil. Therefore, a need exists fora more accurate method of determining the depth to the saturated soil inthe ground. Further, a need exists for a system capable of performingsuch a method.

SUMMARY OF THE INVENTION

The present invention provides a method of determining the depth to thesaturated soil in the ground at a selected site. In one embodiment, themethod includes identifying a first and second instance wherein thedepth of the water table equals the depth to the saturated soil at theselected site. The second instance preferably occurs after the firstinstance. Next, the method determines the depth of the water table atthe first and second instances and the total precipitation that occurredbetween the first and second instances. The rate of change of the depthto the saturated soil may be calculated as a function of the specificyield for the selected site, the amount of time between the instances,the total precipitation that occurred between the instances, and thedepth of the water table at each instance.

With this information, the depth to the saturated soil for a thirdinstance may be calculated. After selecting a third instance, the depthto the saturated soil for the third instance may be calculated as afunction of the rate of change of the depth to the saturated soil.Further, the total amount of precipitation occurring on the thirdinstance and the total precipitation that occurred between the first andthird instances may be determined so that the depth to the saturatedsoil may be adjusted for the third instance as a function of the totalprecipitation that occurred between the first and third instances andthe total amount of precipitation occurring on the third instance. Ifthe third instance occurs after the second instance, the depth to thesaturated soil may be adjusted for the third instance as a function ofthe total precipitation that occurred between the second and thirdinstances and the total amount of precipitation occurring on the thirdinstance.

As another aspect of the present invention, the rate of change of thedepth to the saturated soil may be correlated with factors that affectwater depletion. This correlation may yield one or more depletioncharacteristics of a site that may be used to improve the determinationof the depth to the saturated soil. Specifically, the rate of change ofthe depth to the saturated soil may be correlated with the factors thateffect water depletion to improve the determination of the depth to thesaturated soil at the third instance and/or an instance not between thefirst and second instances.

A system and computer-readable medium capable of performing actionsgenerally consistent with the method above represent further aspects ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A–E are illustrations of a barrel containing soil grains andwater demonstrating the impact of depleting water from the barrel on thedepths of both the saturated soil and water table;

FIG. 2 is an idealized hydrograph of a wetland;

FIG. 3 is an idealized hydrograph of an upland;

FIG. 4 is a flow diagram illustrative of a method for determining thedepth to the saturated soil at a particular site in accordance with thepresent invention;

FIG. 5 is a flow diagram illustrative of a method of preparing a sitefor data collection to determine the depth to the saturated soil at thesite in accordance with the method of FIG. 4;

FIG. 6 is a block diagram depicting a system for collecting site dataand analyzing site data in accordance with the method of FIG. 4;

FIG. 7 is a block diagram depicting an illustrative architecture of adata collection device for use with the system of FIG. 6;

FIG. 8 is a flow diagram illustrative of a data collection methodimplemented by the data collection device of FIG. 7;

FIG. 9 is a block diagram depicting an illustrative architecture of acomputing device capable of implementing a method of determining thedepth to the saturated soil in accordance with the method of FIG. 4;

FIG. 10 is a flow diagram illustrative of a data transfer and analysismethod implemented by the computing device of FIG. 9 in accordance withthe present invention;

FIG. 11 is a flow diagram illustrative of a method of determining thedepth to the saturated soil in accordance with the method of FIG. 4;

FIG. 12A is an exemplary graph of precipitation occurring periodicallyat a selected site;

FIG. 12B is an exemplary hydrograph constructed using the method of FIG.11;

FIG. 13A is an exemplary graph of precipitation occurring at theselected site; and

FIG. 13B is an exemplary graph depicting the '87 Manual standard for awetland at the selected site and the duration of time the soil wassaturated to the surface at the selected site as determined using themethod of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In one aspect of the present invention, a method 100 for determining thedepth to the saturated soil at a particular site, is depicted in FIG. 4.Method 100 starts at block 102. In block 110, a site is selected atwhich the depth to the saturated soil is to be measured. Next, at block120, the site is prepared or properly set up to record measurementsnecessary to determine the depth to the saturated soil.

Referring to FIG. 5, one embodiment of a method 120 of preparing a siteis depicted. First, at least one pit is dug into the ground at the sitein block 122. The pit must be deep enough to detect the depth of thewater table. Preferably, the pit is deep enough to contain water whenthe water table is at its greatest depth. In one embodiment, a suitabledepth is 18 inches.

Next, at block 124, at least one water-level probe is placed in each pitto detect the depth of the water table. Each water-level probe may beconnected to an automatic data collection device. Alternatively, manualrecordation can be taken from the measurements of each water-levelprobe. In one embodiment, water-level probes capable of accuratelydetecting the depth of the water table to within one tenth of an inchare used. However, any suitable water-level probe may be used.Non-limiting examples of suitable water-level probes include pressuretransducers available in stainless steel configurations, water-levelprobes wherein the changing water-level turns a potentiometer, andwater-level probes wherein water-levels are detected from the pressureof air bubbled into the water. Air bubbling probes may consist of an airhose inside a sturdy tube for insertion into the ground. A flange on thetube controls the depth of insertion. A pressure generator such as anaerator pump may be connected to the tube to supply air. Preferably, thepressure generator is located inside an enclosure.

Next, in block 126, a precipitation meter is placed at the site toperiodically measure the amount of precipitation. The precipitationmeter may be connected to an automatic data collection device.Alternatively, manual readings may be taken from the precipitationmeter. Suitable meters include rain gauges or other devices formeasuring the amount of precipitation known in the art. Particularly,water-level probes similar to those discussed above may be used inside ahopper for collecting water provided a means for periodically emptyingthe water from the hopper is included. In one embodiment, an automaticmeans of emptying the water from the hopper is provided.

In block 128, both air and soil temperature probes are installed at thesite. The temperature probes may be connected to an automatic datacollection device or the temperature measurements may be taken manually.Any suitable temperature probe known in the art may be used.Non-limiting examples of suitable temperature probes include commonlyavailable resistant temperature detectors (RTDs) and thermocoupleprobes. At block 129, the site setup method 120 terminates.

Optionally, a hygrometer may be installed to measure humidity at thesite. Any suitable hygrometer known in the art may be used. Thehygrometer may be connected to an automatic data collection device orthe humidity measurements may be taken manually.

Returning to FIG. 4, after the site has been setup in block 120, themethod 100 begins collecting data at block 130. As mentioned above, thevarious probes may be connected to an automatic data collection device.Further, the data collected automatically may be communicated to anotherlocation for further analysis.

Referring to FIG. 6, a system 300 for automatically collecting data fromthe site setup of block 120 is depicted. Each site selected may includea field station 320. Exemplary field station 320A includes threewater-level probes 324A, 324B, and 324C. While three water-level probesare depicted, it is apparent to one of ordinary skill in the art thatmore or fewer water-level probes may be used. Field station 320A alsoincludes a rain gauge 328, an air temperature probe 332, and a soiltemperature probe 334. Some or all of the probes may be connected to anautomatic data collection device. Field stations 320B, 320C, and 320Dmay include probes similar to those of field station 320A.

Referring to FIG. 7, a data collection 400 device suitable forautomatically collecting data from some or all of the water-levelprobes, rain gauge, air temperature probe, and soil temperature probe isdepicted. If automatic data collection is desired, each field station320 may be equipped with a data collection device 400. Data collectiondevice 400 includes a processing unit 410 coupled to a memory 450 via abus 420. Other peripheral devices may also be connected to the processorin a similar manner.

Optionally, data collection device 400 may include a communicationinterface 430 coupled to the processing unit 410 via bus 420.Communication interface 430 may include a cellular telephone modem orother telecommunication or computer communication device known in theart that is capable of transmitting data to the data analysis site 310(shown in FIG. 6). The communication interface 430 may include a modemfor connecting directly to a computer, another data collection device400, or to an Internet service provider through a Point-to-PointProtocol (“PPP”) connection or a Serial Line Internet Protocol (“SLIP”)connection as known to those skilled in the art. The modem may utilize atelephone link, cable link, wireless link, Digital Subscriber Line orother types of communication links known in the art. In anotherembodiment, the communication interface 430 may include a networkinterface for connecting directly to a LAN or a WAN, or for connectingremotely to a LAN or WAN. Those of ordinary skill in the art willappreciate that the communication interface includes the necessarycircuitry for such a connection, and is also constructed for use withvarious communication protocols such as the TCP/IP protocol, theInternet Inter ORB Protocol (“IIOP”), and the like. The communicationinterface 430 may utilize the communication protocol of the particularnetwork configuration of the LAN or WAN it is connecting to, and aparticular type of coupling medium.

Memory 450 may include operating system 455, a data transfer module 460,and a data collection module 470. The operating system 455 may controlthe operation of the data collection device 400. Data transfer module460 includes all of the code and commands necessary to transfer datafrom the data collection device 400 to the data analysis site 310 viathe communication link 314. Data collection module 470 includes all ofthe code and commands necessary to receive measurements from the variousprobes connected to data collection device 400. Additionally, datacollection module 470 may include code for converting the signalsreceived from the field probes for data transfer or to place them in aformat that is more easily accessible by users or other softwareprograms.

FIG. 8 depicts one embodiment of a method 500 that the data collectiondevice 400 may use to collect and transmit data. In one embodiment, thedata analysis site 310 may send a request to transfer data to the fieldstation 320. Referring to FIG. 8, when the data collection device 400receives a request to transfer data at block 510, the data transfermodule 460 receives the request. Next, the data collection module 470obtains the measurements from the various probes. At this point, thedata collection module 470 may need to convert or compress the data fortransfer. The data collected is then transferred over communication link314 to the data analysis site 310 by the data transfer module 460.

Alternatively, data collection device 400 may include a data recordationmodule that includes the computer-readable components required to recordmeasurements taken from the various probes in memory 450. Memory 450 mayinclude removable media such as removable storage drives, writable CDs,and floppy discs that can be removed and transferred to another locationsuch as data analysis site 310. In another embodiment, data collectiondevice 400 may include a data communication interface that allows a userto download data stored in memory 450 directly to another device orexternal memory. In this embodiment, data can be analyzed on site ortransferred elsewhere.

Data collection device 400 may be located inside an enclosure or mayinclude its own water-tight housing. The power supply for datacollection device 400 may include solar panels, micro wind turbines,and/or batteries. Alternatively, if data collection device 400 islocated inside an enclosure, the device may be connected to a typicalelectrical wall outlet. While a standard personal computer may performthe functions of data collection device 400, a computer is notnecessarily required, other devices such as programmable logiccontrollers and embedded systems may be used to collect and transmit thedata collected.

It may be beneficial to connect more than one field station 320 to acentralized data analysis site 310. As depicted in FIG. 6, multiplefield stations 320A, 320B, 320C, and 320D, may be connected to dataanalysis site 310 via communications links 314A, 314B, 314C, and 314D,respectively. In this manner, as the data collection device 400 at eachfield station collects data, it may be transferred to data analysis site310. Data analysis site 310 may include a computing device 600 forreceiving and analyzing data from the field stations 320.

In one embodiment, the data analysis site 310 includes a computingdevice 600, such as a personal computer. FIG. 9 depicts several of thekey components of the computing device 600. Those skilled in the artwill appreciate that the computing device includes many more componentsthan are shown in FIG. 9. However, it is not necessary that all of thesegenerally conventional components be shown in order to disclose anenabling embodiment for practicing the present invention.

The computing device 600 includes a processing unit 610, a display 640,and a memory 650. The memory 650 generally comprises a random accessmemory (“RAM”), a read-only memory (“ROM”), and a permanent mass storagedevice, such as a hard disk drive, tape drive, optical drive, floppydisk drive, CD-ROM, DVD-ROM, or removable storage drive. The memory 650stores an operating system 655 for controlling the operation of thecomputing device 600. The memory may also store the data transfer module660, problem detection module 670, and data analysis module 680 of thepresent invention. It will be appreciated that the operating system 655and aforementioned modules may be stored on a computer-readable mediumand loaded into memory 650 of the computing device 600 using a drivemechanism associated with the computer-readable medium, such as afloppy, CD-ROM, DVD-ROM drive, or network interface. The memory 650 anddisplay 640 are connected to the processor 610 via a bus 620. Otherperipherals may also be connected to the processor in a similar manner.

As shown in FIG. 9, in one embodiment of the invention, the computingdevice 600 may include a communication interface 630 coupled to theprocessor 610 for establishing a communication link with externalcomputers and computing networks. The communication interface 630 mayinclude a modem for connecting directly to another computer, the datacollection device 400, or to an Internet service provider through aPoint-to-Point Protocol (“PPP”) connection or a Serial Line InternetProtocol (“SLIP”) connection as known to those skilled in the art. Themodem may utilize a telephone link, cable link, wireless link, DigitalSubscriber Line or other types of communication links known in the art.In another embodiment, the communication interface 630 may include anetwork interface for connecting directly to a LAN or a WAN, or forconnecting remotely to a LAN or WAN. Those of ordinary skill in the artwill appreciate that the communication interface includes the necessarycircuitry for such a connection, and is also constructed for use withvarious communication protocols, such as the TCP/IP protocol, theInternet Inter ORB Protocol (“IIOP”), and the like. The communicationinterface may utilize the communication protocol of the particularnetwork configuration of the LAN or WAN it is connecting to, and aparticular type of coupling medium.

Memory 650 may include a data transfer module 660 for receiving datafrom each field station 320 via communications link 314. In oneembodiment, memory 650 also includes a problem detection module 670.Problem detection module 670 includes computer code and componentsnecessary to detect when the data received from a particular fieldstation 320 contains errors. For example, if a quick movement of thewater-level occurs without the onset of a rainstorm, there may be aproblem in the system. Additionally, the problem detection module 670may alert a user if a particular field station is not transmitting allof the data it should be collecting.

Additionally, memory 650 may include a data analysis module 680. Dataanalysis module 680 may include all of the calculations, functions, andcomputer code necessary to perform the method of determining the depthto the saturated soil of the present invention. The method 140 ofdetermining the depth to the saturated soil will be described in greaterdetail below with reference to FIG. 11.

Referring to FIG. 10, one embodiment of a method by which the computingdevice 600 may receive and analyze data is depicted. The method 700starts at block 701. Next, the data transfer module 660 requests datafrom at least one field station 320. In one embodiment, such requestsoccur automatically every 120 seconds. After data is received, theproblem detection module 670, examines the data and determines whetherto issue a warning of a problem with the data. At decision block 710,the user decides whether to analyze data. If the user wishes to analyzedata, the data analysis module 680 performs the analysis. If the userdecides not to analyze data or has finished analyzing data, the userdecides whether to transfer additional data at decision block 720. Ifthe user wishes to transfer additional data, the method 700 returns tothe data transfer module 660. Otherwise, the method 700 terminates atblock 799.

After sufficient data has been collected in block 130, the data may beanalyzed to determine the depth to the saturated soil in the ground at aparticular site in block 140. Referring to FIG. 11, the method 140 foranalyzing the data collected is illustrated in greater detail. Thismethod may be performed manually or by automatic means such as dataanalysis module 680. The method 140 starts at block 141. Two instanceswhere the depth to the saturated soil equals the depth of the watertable are selected as a first and second instance in block 142.Preferably, there are no instances in which the depth to the saturatedsoil equals the depth of the water table occurring between the first andsecond instance.

An instance where the depth to the saturated soil equals the depth ofthe water table can be identified in at least three ways. First,whenever the water table is at the surface of the soil, the depth to thesaturated soil is also at the surface of the soil. Second, whenever thewater table is above the surface of the soil, the depth to saturatedsoil is also above the surface of the soil. Lastly, whenever there issufficient rain, the depth of the water table will rise rapidly to equalthe depth to the saturated soil. Therefore, a rapid rise in the depth ofthe water table coinciding with rain fall may signify that the depth ofthe water table and the depth to saturated soil are equal. Therefore,instances identified as occurring during at least these threecircumstances may be used as the first and/or second instances.

In block 146, the rate of change of the depth to the saturated soil iscalculated between the first and second instances. The rate of change ofthe depth to the saturated soil may be calculated as a function of theamount of time between the first and second instances, the depth of thewater table at the first and second instances, the total precipitationoccurring between the first and second instances, and the specific yieldof the soil at the site. Specifically, the rate of change of the depthto the saturated soil may be calculated as follows. First, the totalprecipitation occurring between the first and second instances isdivided by the specific yield of the soil at the site to obtain offsetα. Because determining specific yield may be difficult, an assumed valuesuch as 0.3 may be used. If an assumed or initial value is used, thevalue may be modified later as more data is collected to better reflectwater depletion at the site. Next, offset α is added to the depth of thewater table at the second instance and the depth of the water table atthe first instance is subtracted from that sum to obtain depth β. Then,depth β is divided by the amount of time between the first and secondinstances to obtain the rate of change of the depth to the saturatedsoil. In some embodiments, it may be beneficial to measure time inperiods that are easily graphed. In those embodiments, depth β maysimply be divided by the number of periods between the first and secondinstances to acquire the rate of change of the depth to the saturatedsoil. The rate of change value may be stored such as in memory 650 forfuture use.

In block 148, the depth to the saturated soil is calculated for aninstance occurring between the first and second instances. On ahydrograph, where the X-axis is time and the Y-axis is depth, the firstand second instances and the depth of the water table at those instancesmay be plotted. By drawing a line from the first instance with a slopeequal to the rate of change of the depth to the saturated soil, thedepth to the saturated soil can be estimated for instances occurringafter the first instance and before the second instance. In other words,a linear model of the depth to the saturated soil can be produced. Whilein reality depletion mechanisms such as drainage, evaporation, and/ortranspiration may not deplete the water from the ground linearly, alinear model offers a reasonably close approximation of the depth to thesaturated soil. However, as more data is collected, it may be desirableto use the data collected to calculate a non-linear model of the rate ofchange of the depth to the saturated soil.

In the preferred embodiment, the linear model is adjusted to include theeffects of precipitation occurring at the site. The adjustment is basedon the general principles of conservation of mass. After an instancebetween the first and second instances is selected, the adjustment iscalculated by totaling all of the precipitation occurring from the firstinstance, to and including the precipitation occurring on the selectedinstance. Using the depth of the water table at the first instance andthe rate of change of the depth to the saturated soil, the unadjustedpredicted depth to the saturated soil may be obtained. Then, theadjustment (i.e., the total precipitation occurring from the firstinstance to and including the precipitation occurring on the selectedinstance) may be added to the unadjusted predicted depth. On ahydrograph, like the one discussed above, the adjustment may be depictedat the periods where precipitation occurred. Specifically, at a periodwhere rain occurred, the linear model may be “stepped up” by an amountcorresponding to the amount of precipitation that occurred on thatperiod. In one embodiment, the depth of the linear model is simplystepped up (or moved closer to the surface of the soil) by an amountequal to the amount of precipitation occurring on that period. This maygive the hydrograph a saw-toothed appearance.

FIGS. 12A and 12B illustrate a hydrograph constructed in accordance withthe present invention. FIG. 12A depicts a precipitation graph 820. Thisgraph may be useful to determine the total precipitation between thevarious instances and the periods at which precipitation occurred.

Referring to FIG. 12B, a non-limiting example of a hydrograph producedin accordance with the present invention is provided for illustrativepurposes. Hydrograph 840 includes two instances 850 and 852 where thedepth to the saturated soil equals the depth of the water table becauseat both the first instance 850 and the second instance 852 the watertable was at the surface of the soil 844. Consequently, points 850 and852 represent instances where both the water table 842 and the depth tothe saturated soil were at the surface of the soil 844. It is preferredto use instances where the water table and the depth to the saturatedsoil are at the surface of the soil because this fact can be easilyverified either visually or by water-level probes. The water-levelprobes 324 located at each field station 320 may be used to measure thedepth of the water table over time. On hydrograph 840, the depth of thewater table over time is depicted as line 842. The offset α wascalculated as described above and is depicted as line 860. The offset αwas then added to the depth of the water table at the second instance toobtain point 862. Next, the rate of change of the depth to the saturatedsoil is depicted as the slope of line 870 extending from the firstinstance 850 to point 862. In other words, the rate of change of thedepth to the saturated soil can be determined by connecting the firstinstance to a point located offset distance a below the second instance.

As can be seen in hydrograph 840, the linear model (i.e., line 870) isnot the best predictor of the depth to the saturated soil because inseveral locations, the line 870 passes beneath the water table. Inreality, the depth to the saturated soil is always at or above the depthof the water table. Therefore, adjustments based on precipitation may benecessary.

The periodic amounts of precipitation measured by the precipitationmeter 328 are depicted in the precipitation graph 820. To adjust thehydrograph, periods in which precipitation occurred between the firstand second instances must be identified. In one embodiment, periods inwhich only trace amounts of precipitation occurred may be ignored.Ignoring periods with trace amounts of precipitation, there are sixperiods between the first 850 and second 852 instances in whichprecipitation occurred: March 8, March 9, March 10, March 15, March 16,and March 21. On each of these periods, the adjusted linear model 880representing the depth to the saturated soil is stepped up by the amountof precipitation occurring on the period. Specifically, referring toMarch 8, a vertical line with a length H2 equal to the amount ofprecipitation H1 that fell on Mar. 8, 1998, is drawn from theintersection of the middle of Mar. 8, 1998, and line 870. The linearmodel is then continued until the next period with rain is encountered,where an adjustment may again be performed. These steps are repeateduntil the hydrograph reaches the second instance 852.

After the second instance 852 is reached, the data analysis process 140may be repeated for the next pair of instances where the depth of thewater table equaled the depth to the saturated soil. Referring to FIG.12B, the next pair of instances may include 852 and 854; 854 and 856; or856 and 858. It is important to note that the depth of the water tablewas above the surface of the soil the entire time between instances 852and 854. Therefore, the data analysis method 140 need not be performedon this pair of instances.

Referring to FIG. 12B, it can be observed that in two instances 856 and858 the depth to the saturated soil equaled the depth of the water tablebelow the surface of the soil. These instances may be treated the sameas instances where the water table is at the surface of the soil.Further, these instances may be used to test the accuracy of thehydrograph. If the hydrograph predicted the depth of the water tablewould equal the depth to the saturated soils within a reasonable degreeof error, the model may not require adjustment. However, if the degreeof error is too large, the rate of change of the depth to the saturatedsoil may be modified or a non-linear rate of change applied. As anon-limiting example, an error of up to one inch may be used.

In one embodiment of the present invention, the rate of change of thedepth to the saturated soil is calculated for each pair of instancesselected. For example, the slope of line 872 represents the rate ofchange of the depth to the saturated soil between instances 854 and 856.Referring to FIG. 12B, it may be observed that the slope of line 870does not equal the slope of line 872.

The rate of change of the depth to the saturated soil can be used topredict future the depths of the saturated soil. Further, the rate ofchange can be averaged over time to create a more accurate prediction ofthe depth to the saturated soil. In other words, because the rate ofchange of the depth to the saturated soil corresponds to the rate atwhich water is depleted from the site, the rate of change of the depthto the saturated soil can be used as a predictor of the rate of futurewater depletion from the site. Additionally, the total amount ofprecipitation occurring from the second instance, to and including theprecipitation occurring on the future instance may be used to adjust thepredicted depth to saturated soil at the future instance.

Generally, the rate of depletion may be affected by several factors.Specifically, at least the following factors may affect the rate ofdepletion: the depth to the saturated soil, the temperature of the soil,the temperature of the air, humidity, seasonal changes in vegetation, inflow of water from the watershed above the site, and outflow of thewater to areas downstream of the site. Because the depletion rate mayvary with these factors, these factors may be correlated with the rateof change of the depth to the saturated soil to create improvedpredictions or a set of depletion characteristics. For example,referring to FIG. 12B, the slope of line 870 is less than the slope ofline 872. This difference in slope may be the result of one or more ofthe factors that affect the rate of depletion. Therefore, correlatingeach of these slopes with one or more of the factors that affect therate of depletion may improve the model (hydrograph) of the site. Therate of change of the depth to the saturated soil and any correlationwith other information or factors may be stored such as in memory 650for future use.

Because several of the above factors may vary over time and with theseasons, the depletion rate may also vary over time. Therefore, it maybe beneficial to determine one or more depletion rates over the courseof a year or for several years. In other words, data collected over timemay be used to create an improved set of depletion characteristics for asite that may be used to determine and predict the depth to thesaturated soil.

Additional circumstances may also effect the depletion rate. Forexample, changes to the topography or surface water of surrounding landsmay change the depletion characteristics of a site. Therefore, sites mayrequire occasional reevaluations or continuous monitoring. Particularly,sites that border on being considered wetlands may benefit from suchmeasures.

To verify accuracy, the present invention is used to predict what thesaturation level will be the next time it rains. This initial predictionis based on data collected and analysis carried out previously, asdiscussed above. Unless the predicted saturation level is accurate, thepresent invention utilizes different equations stored in memory 450until the processing unit 410 locates an equation or equation set thatwill give the correct results. Many equations are stored in memory 450,and likely more than one of these equations will be satisfactory thefirst time the saturation level analysis is carried out. Later, whenmore data is available, the processing unit 410 may have to search for adifferent equation that works not only with the initial or previous datacollected, but also with newer data. This process of selecting newequations to fit the data collected over time may occur at least severaltimes. These equations, as noted above, account for many variables andfactors, such as depth of water table, air and soil temperature,humidity, inflow and outflow of water to areas above and below the testsite, wind speed and direction, types and quantities of vegetation, timeof year (which is relevant to whether the vegetation is active ordormant), and other factors. The end goal is to develop a model of asoil-water system where the saturation level accurately corresponds toprecipitation inputs. It may take a year or more to create an accuratemodel for a particular test site.

A basic equation to predict future depth to saturated soil level h3 attime t3 from prior levels h1 and h2 at prior times t1 and t2 is:h3=h2+H(2-to-3)−(h2−h1)(t3−t2)/(t2−t1)

where: h3 is the predicted depth to saturated soil level at time t3

-   -   h1 is the depth to saturated soil level at time t1    -   h2 is the depth to saturated soil level at time t2    -   H(2-to-3) is the rise in soil water due to precipitation in the        interval t2 to t3

The expression:h(h2−h1)/(t2−t1)is the rate of water depletion in the time period from t1 to t2, andH(2-to-3)=P(t2-to-t3)(X)

where: P(t2-to-t3) is the precipitation in the period t2 to t3, and

-   -   X is the “effective” increase in water volume from outside to        inside the soil.

For the same amount of precipitation, field data for levels h2 and h3vary relative to h1 according to various factors: depth below thesurface, temperature outside and inside the soil, wind, humidity, inflowand outflow, vegetation, etc. In the mathematical model of thesoil-water system, the processing unit 410 substitutes depletion ratesuntil it arrives at the measured value for h3.

The foregoing adaptive measuring system utilized the phenomena ofdifferential pressure fronts. As discussed above, a negative pressurezone or a “capillary fringe” requires a “seal” against atmosphericpressure. The seal begins at the water table and extends over anindefinite area before returning to the water table. Together, capillarymenisci form differential pressure fronts. The magnitude of the pressuredifferential across them depends on the size of the soil grains, i.e.,smaller grains means higher negative pressures.

A front must be continuous to create a negative pressure and itincorporates rocks, roots and non-homogeneous soils. In a dug pit, thefront follows the wall down to the water table, where the pressure isatmospheric, and back up on the other side. Like a front, the watertable is an irregular surface. At each point it slopes in proportion tothe drain rate; it is not horizontal if the soil drains.

A capillary fringe only exists between a front and a water table. Itdepends on the front's ability to generate a pressure differential andcan extend over acres of land. A shovel cannot puncture a front like aneedle pops a balloon; however, a few drops of water will. A smallincrease in water volume below the front relaxes the capillary menisciand eliminates the pressure differential.

With the front gone, the water level in a pit will rise to thesaturation level as fast as water can seep in. he body of water in thecapillary fringe cannot move instantaneously; it remains in place exceptit is no longer a negative pressure zone. The water that made up thefringe is no longer suspended from the soil grains along the front.

The foregoing mathematical model can be tested when the water tablelevel equals the saturation level to compare the actual depth tosaturated soil to the predicted level using the current math model. Ifthe delta is not insignificant, the processing unit 410 constructs a newmathematical model of the soil-water system of the site from the sets ofpreviously determined and stored equations. The processing unit 410matches the model's and the actual soil system's response toprecipitation; predicted and measured saturation levels at the onset ofrain have to be the same.

As noted above, at first, several mathematical models will work;however, soil-water depletion rates vary and what works during thewinter may not work during the summer. Therefore, an acceptable modelmust be a composite that works throughout the year—accounting for depthbelow the surface, air and soil temperature, humidity, inflow andoutflow, wind, vegetation, etc.

Returning to FIG. 4, after the analysis of data has completed, theresults can be compared with standards to determine whether a sitequalifies as a wetland in block 170. Referring to FIGS. 13A and 13B, anexample of such of comparison is depicted. FIG. 13A depicts the averagemonthly precipitation occurring in a 12 month period at a specific site.Area 920 illustrates the average precipitation occurring each month.Line 924 indicates the start of the growing season and line 926indicates the end of the growing season.

FIG. 13B illustrates a time-span graph depicting two data sets. Thehorizontal lines 940 and 950 illustrate the time-spans of each data set.Lines 940 illustrate the first data set. The first data set isconstructed from an interpretation of text of the '87 Manual and depictsa permissible pattern of saturation to the surface of the soil that canoccur before a site is a wetland. None of the time-spans depicted aslines 940 equal 5% of the growing season.

The second data set is the amount of time during the growing season thatthe soil at a particular site was saturated to the surface. Lines 950illustrate the periods during March that hydrograph 840 demonstratedthat the surface of the soil was saturated. Specifically, lines 950correspond to the period before and up to instance 850 and the periodbetween instances 852 and 854 of FIG. 12B. FIG. 13B clearly illustratesthat none of the lines 950 continuously occupy 5% of the growing season.Therefore, according to the '87 Manual, the site depicted in FIGS.12A–13B is not a wetland.

Returning to FIG. 4, in decision block 180, a decision is made whetherto continue monitoring the site. If monitoring is continued, method 100returns to collecting data in block 130. If monitoring is not continued,the setup is removed from the site at block 190 and the methodterminates at block 199.

The method described herein and the system for implementing the methodprovide an objective means by which the depth to the saturated soil maybe determined. It may be desirable to adjust the calculation parameters,such as specific yield, to achieve a particular level of accuracy. Forexample, it may be desirable to achieve an accurate prediction of thedepth to the saturated soil up to one inch.

As mentioned above, it may be desirable to correlate the rate of changeof the depth to the saturated soil with the factors that affect waterdepletion. In this manner, a set of depletion characteristics can beobtained that may be used to improve the accuracy of the determinationand/or prediction of the depth to the saturated soil.

The system removes much of the necessity to make numerous visits to thesite. Further, the system provides an opportunity to measure the depthto the saturated soil continuously over a long duration of timeincluding more than one growing season. The more data collected the moreaccurate the hydrograph may become as it is refined over time becausethe accuracy of predictions based on the hydrograph can be verified withthe next rainfall and the depletion characteristics (including the rateof change of the depth to the saturated soil) adjusted accordingly.Additionally, factors that vary over time may be considered to create ahydrograph that includes variation created by such factors.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method of determining a depth to the saturated soil in the groundat a selected site comprising: a. identifying a first instance and asecond instance wherein a depth of the water table equals a depth to thesaturated soil at the selected site and the second instance occurs afterthe first instance; b. determining the depth of the water table at thefirst and second instances at the selected site; c. determining a totalprecipitation that occurred between the first and second instances atthe selected site; d. calculating a rate of change of the depth to thesaturated soil as a function of i. a specific yield value for theselected site, ii. an amount of time between the first and secondinstances, iii. the total precipitation that occurred between the firstand second instances, and iv. the depth of the water table at the firstand second instances; e. formulating an initial equation that models thedepth to the saturated soils between the time period of the first andsecond instance; f. using the initial equation to calculate the depth tothe saturated soil for a third instance; g. measuring the depth to thesaturated soil for a third instance; h. determining the differencebetween the calculated and measured depth to saturated soil for thethird instance; i. if the difference between the calculated and measureddepth to saturated soil for the third instance exceeds a predeterminedamount, formulating a new equation that models the measured depth tosaturated soil for the third instance.
 2. The method of claim 1,comprising correlating the depth to the saturated soil at the thirdinstance with a temperature of the soil at the selected site.
 3. Themethod of claim 1, comprising correlating the depth to the saturatedsoil at the third distance with a temperature of the air at the selectedsite.
 4. The method of claim 1, comprising correlating the depth to thesaturated soil with time duration between the second and thirdinstances.
 5. The method of claim 1, comprising correlating the depth tothe saturated soil at the third instance with the humidity at theselected site.
 6. A system for determining a depth to the saturated soilin the ground at a selected site, comprising: a. a means forperiodically measuring an amount of precipitation at the selected siteto obtain a series of precipitation measurements; b. a means for storingthe series of precipitation measurements; c. a means for periodicallymeasuring the depth of the water table at the selected site to obtain aseries of depth of water table measurements; d. a means for storing theseries of depth of water table measurements; e. a means for identifyinginstances within the stored series of depth of water table measurementswherein the depth to the saturated soil equals the depth of the watertable; and f. a means for determining a rate of change of the depth tothe saturated soil between a pair of instances identified as instanceswhere the depth to the saturated soil equals the depth of the watertable as a function of a specific yield value for the selected site, aportion of the stored series of precipitation measurements occurringbetween the pair of instances, and a portion of the stored series ofdepth of water table measurements corresponding to each instance in thepair of instances. g. means of formulating an equation to predict thedepth to the saturated soil as a function of selected factors; h. meansfor determining the difference between the predicted depth and thecritical depth of saturated soil; and i. means for formulating a newequation to predict the depth to the saturated soil that takes intoconsideration the difference between the predicted and actual depths tosaturated soil.